core/rawdb: implement tail truncation change in memory freezer

core/rawdb/ancienttest/testsuite.go

@@ -260,6 +260,24 @@ func basicWrite(t *testing.T, newFn func(kinds []string) ethdb.AncientStore) {
 	if err != nil {
 		t.Fatalf("Failed to write ancient data %v", err)
 	}
+
+	// Write should work after truncating from tail but over the head
+	db.TruncateTail(200)
+	_, err = db.ModifyAncients(func(op ethdb.AncientWriteOp) error {
+		offset := uint64(200)
+		for i := 0; i < 100; i++ {
+			if err := op.AppendRaw("a", offset+uint64(i), dataA[i]); err != nil {
+				return err
+			}
+			if err := op.AppendRaw("b", offset+uint64(i), dataB[i]); err != nil {
+				return err
+			}
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatalf("Failed to write ancient data %v", err)
+	}
 }
 
 func nonMutable(t *testing.T, newFn func(kinds []string) ethdb.AncientStore) {

core/rawdb/freezer_memory.go

@@ -113,7 +113,7 @@ func (t *memoryTable) truncateTail(items uint64) error {
 		return nil
 	}
 	if t.items < items {
-		return errors.New("truncation above head")
+		return t.reset(items)
 	}
 	for i := uint64(0); i < items-t.offset; i++ {
 		if t.size > uint64(len(t.data[i])) {
@@ -127,6 +127,16 @@ func (t *memoryTable) truncateTail(items uint64) error {
 	return nil
 }
 
+// reset clears the entire table and sets both the head and tail to the given
+// value. It assumes the caller holds the lock and that tail > t.items.
+func (t *memoryTable) reset(offset uint64) error {
+	t.size = 0
+	t.data = nil
+	t.items = offset
+	t.offset = offset
+	return nil
+}
+
 // commit merges the given item batch into table. It's presumed that the
 // batch is ordered and continuous with table.
 func (t *memoryTable) commit(batch [][]byte) error {